Differential pressure transducer

ABSTRACT

A DIFFERENTIAL PRESSURE TRANSDUCER OF THE CAPICATOR TYPE UTILIZING ISOLATION DIAPHRAGMS FOR SENSING THE DIFFERENTIAL PRESSURE DIRECTLY, AND HAVING A CENTRAL CHAMBER WITH A SENSING DIAPHRAGM IN THE CENTRAL CHAMBER. THE CENTRAL CHAMBER IS FILLED WITH OIL OR A SUITABLE LIQUID WHICH TRANSMITS PRESSURE FORM THE ISOLATION DIAPHRAGMS TO THE SENSING DIAPHRAGM. THE SENSING DIAPHRAGM FORMS A CAPACITOR PLATE, AND THE WALLS OF THE CHAMBER ADJACENT TO THE SENSING DIAPHRAGM ALSO FROM CAPACITOR PLATES. THE PRESSURES ARE TRANSMITTED TO THE SENSING DIAPHRAGM THROUGH THE OIL SO THAT ONLY DIFFERENTIAL PRESSURE IS SENSED BETWEEN THE TWO ISOLATION DIAPHRAGMS. THE SENSING DIAPHRAGM AND ITS CHAMBER ARE MADE SO THAT ANY EXCESSIVE PRESSURE CAUSES THE SENSING DIAPHRAGAM TO BOTTOM OUT AGAINST ITS MOUNTING CHAMBER SURFACES. THE UNIT IS METAL EXTREMELY STABLE, BY USING A RELATIVELY MASSIVE METAL HOUSING WITH A GLASS CENTER PORTION PLATE MATERIAL RIGHT DEPOSITING THE METALLIC CAPACITOR PLATE MATERIAL RIGHT ONTO THE GLASS SURFACES WHICH FORM THE SENSING CHAMBER.

Nov. 9, 1971 R. L. FRICK DIFFERENTIAL PRESSURE TRANSDUCER 2 Sheets-Sheet 1 Filed Oct. 21 1969 READ OUT Nov. 9, 1971 R. L. FRICK 3,613,390

DIFFERENTIAL PRESSURE TRANSDUCER Filed Oct. 27, 1969 2 Sheets-Sheet :1

33 I45 INVIiNIUR. 36 3 ng/ FOzSQLFE/CK 47' TOE NE Y5 U.S. Cl. 73398 C 22 Claims ABSTRACT OF THE DISCLOSURE A differential pressure transducer of the capacitor type utilizing isolation diaphragms for sensing the differential pressure directly, and having a central chamber with a sensing diaphragm in the central chamber. The central chamber is filled with oil or a suitable liquid which transmits pressure from the isolation diaphragms to the sensing diaphragm. The sensing diaphragm forms a capacitor plate, and the walls of the chamber adjacent to the sensing diaphragm also form capacitor plates. The pressures are transmitted to the sensing diaphragm through the oil so that only differential pressure is sensed between the two isolation diaphragms. The sensing diaphragm and its chamber are made so that any excessive pressure causes the sensing diaphragm to bottom out against its mounting chamber surfaces. The unit is made extremely stable, by using a relatively massive metal housing with a glass center portion fused thereto, and depositing the metallic capacitor plate material right onto the glass surfaces which form the sensing chamber.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to differential pressure transducers which are of the capacitive type.

(2) Prior art U.S. Pat. No. 3,232,114 shows a differential pressure transducer of the capacitive type wherein the fluid which is being sensed for pressure acts directly upon at capactive sensing diaphragm. This does not show the use of isolation diaphragms which contact the fluid to be measured for pressure or the use of a liquid to transmit the pressure to a sensing diaphragm. U.S. Pat. No. 3,342,072 also shows a type of pressure measuring device which utilizes a liquid filled chamber, but which has a different construction of the sensing units. Other United States patents which disclose differential sensing capabilities include U.S. Pat. Nos. 3,295,326; 3,350,945; 3,372,594; 3,258,971 and 3,158,000.

None of these structures embody the features of the present device which includes capacitive sensing with the capacitor plates isolated from the fluid being sensed, positive overpressure protection, and very stable sensing characteristics.

SUMMARY OF THE INVENTION The present invention relates to a pressure transducer using capacitive sensing with one portion of the capacitor formed by depositing a metal layer onto a glass or ceramic surface fused to a large metal housing. A differential pressure transducer for sensing differential pressures between two sources of fluid and which isolates the fluid from the sensing portion of the unit is shown. The transducer is provided with isolation diaphragms, and there is a liquid transferring pressure from the isolation diaphragms to the opposite sides of a measuring or, sensing diaphragm so that any pressure on one isolation diaphragm different from the pressure on the opposite isolation diaphragm will cause the sensing diaphragm fnited States Patent '01 3,618,390 Patented Nov. 9, 1971 to move relative to sensing capacitive plates. Overpressure capabilities are provided by designing the unit so that the measuring diaphragm will bottom out and be supported against the walls forming its chamber when subjected to overpressure, and where the chamber housing itself is very stable to prevent shifts in calibration when the unit has bottomed out. The unit is designed so that full scale deflection is only a few pounds per square inch, while the unit will withstand differential overpressures of several thousand pounds per square inch without major shifts in calibration.

It is therefore an object of the present invention to present a stable differential pressure transducer which is easy to make, economical in operation and remains accurate even after being subjected to large overpressures.

The invention further includes the concept of making a pressure sensor which uses an isolation diaphragm, a liquid to transfer pressure to a sensing diaphragm, and a mechanical stop for the sensing diaphragm which stops the sensing diaphragm from movement before the isolation diaphragm is mechanically stopped.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of the pressure transducer capsule utilizing a sensor made according to the present invention;

FIG. 2 is a sectional view taken as on line 22 in FIG. 1;

FIG. 3 is a sectional view taken as on line 33 in FIG. 2;

FIG. 4 is an enlarged sectional view of the sensing diaphragm and isolation diaphragms of the present invention, and

FIG. 5 is a fragmentary sectional view taken as on line 55 in FIG. 2

DESCRIPTION OF THE PREFERRED EMBODIMENT A differential pressure transducer assembly unit indicated generally at 10 includes a main pressure transducer housing 11, and a sensing electronic circuitry housing shown fragmentarily at 12. The circuitry is shown schematically at 12A. The two units are connected together with a connecting element 13. The pressure transducer housing, as shown, includes a first end cap 14, a center housing section 15, and a second end cap 16 which are fastened together in an assembly with suitable cap screws 17. Suitable conduits 18 and 19, respectively, open into the end caps. End cap 14 has an interior chamber 21 centrally located therein and the end cap 16 has an interior chamber 22. The conduit 19 opens to chamber 22 and the conduit 18 opens to chamber 21. The sources of fluid to be sensed are connected to the respective conduits and differential between the pressures of these two fluids will be sensed in the center housing section 15. The fluids can be gaseous or liquid and may be highly corrosive, without damaging the sensing unit.

As shown, the end caps are hemispherically shaped, and the interior chambers are circular in cross section and have a domed upper surface. Suitable passageways containing plugs 24 are provided for purposes of bleeding the chambers, or for servicing.

Center housing section 1 5 mounts the interior sensing cell assembly 26 which is also shown in detail in FIG. 4. The interior sensing cell assembly includes a large metal housing 27 that in initial stages of manufacture is made into two sections 27A and 27B divided along a parting line Where a measuring or sensing diaphragm 28 is placed.

0 The metal member halves 27A and 27B have cup-like glass, which is used in the form disclosed, is placed into cavities 29 and fused to the metal walls of sections 27A or 27B in a furnace. Besides the main glass filled cavities 29, the sections each have a central passageway 32, and an opening 30 which open to a depression or shallow cavity 31 on the opposite side of each housing section from the glass filled cavities 29. Each opening 30 communicates with its corresponding passageway 32. A separate ceramic tube 33 having a plurality of axially extending openings 36 is placed into each of the passageways 32 and the ceramic tubes extend into the cavities 29. The ceramic tubes are positioned in the cavities before the glass is fused, and the tubes 33 are fused in place. Small metal tubes 34 are placed through provided openings in the wall of each of the sections of the housing 27 and extend into the cavities 29. The tubes 33 and 34 actually extend out beyond the edges of the sections 27A and 27B initially so they extend out beyond the glass after the cavities 29 have been filled and fused. Note that the glass insulates the metal tubes 34 where they pass out through the side walls of the housing sections.

Then each section of the housing is ground off and a concave surface which is, in part, substantially spherical indicated at 38 is ground into the glass in the cavities. The tubes 33 and 34 are ground off at the same time so that the tubes form a continuation of the concave surface. Then the surfaces 38 of the unit are each covered with a thin metal coating. These are indicated at 39 and 40, respectively, and form capacitor plates when assembled. The depth of each concavit is shown greatly exaggerated and typically is between .003 and .010 inch as measured at the center, where the depth is maximum relative to the edge. The diameter of the diaphragm 28 is typically between .3 and 3.0 inches. The thickness of insulating section 29 is typically greater than .10 inch in order to maintain sufficiently low shunt capacitance between electrodes subsequently formed at surfaces 38 and metal case 27.

The diameter of passageway 32 is typicall in the order of 0.2 inch and passageways 36 may be .020 inch for example. These passageways must be sufficiently small so that diaphragm 28 is not overly stressed by the presence of those openings when the diaphragm is subjected to maximum overpressures.

In construction, when the two sections 27A and 27B have been prepared as described, the metal diaphragm 28 is then placed between the sections, stretched taut and is welded into place with a continuous bead Weld 41 which joins the two sections 27A and 27B together and holds the diaphragm under a desired amount of stress. The weld 41 also hermetically seals the chambers on the interior of the housing.

First and second isolation diaphragms 44 and 45, respectively, are then welded into place around the peripheries of the depressions or cavities 31 in each of the housing sections. As can be seen, the isolation diaphragms are made of thin stainless steel and are corrugated so they will flex easily.

When assembled, the sensing diaphragm and the isolation diaphragms divide the sensing unit 26 into two completely sealed sensing chambers comprising a first chamber 48 and a second chamber 49. The sensing chambers 48 and 49 are filled with a liquid, which may be a silicone base oil, through their respective tubes 34 which lead into the chambers. After filling, the tubes 34 are sealed at their outer ends. The chambers 48 and 49 are open through the holes 36 in the ceramic tubes 33 to the holes 30 in the sections 27A and 27B, respectively, and then into the cavities 31 defined in the outer surfaces of the metal cups or sections 27A and 27B underneath the respective diaphragms 44 and 45. The diaphragms 44 and 45 are welded all the way around their edges so that this is a completely sealed unit and the diaphragms form walls of isolation chambers in cooperation with the cavities 31. When the pressures in. chambers 21 and 22 are equal, the measuring diaphragm 28 will extend substantially straight across the parting line between the two sections of the metal housing 27.

The oil in chambers 48 and 49 is put in through the filler tubes 34 until the desired volume of oil in each of the sections is reached and the tubes 34 are sealed off. The metal tubes 34 are then used for leads to the respective capacitor plates 39 and 40 and are connected to suitable readout circuitry. The lead to the sensing diaphragm 28 can be fixed to the metal housing as the diaphragm is welded to the housing and the capacitor plates 39 and 40 are insulated from the housing.

In order to keep the pressure from the cap screws 17 off the sensing cell assembly, a unique way of retaining the center sensing cell is utilized. A massive ring 50, which also includes the connecting portion 13 as a unit assembly, is utilized and this has a large center opening, as shown, into which the sensing cell assembly 26 will fit. A pair of annular rings 51 and 52, respectively, are welded onto the outer edges of the housing sections 27A and 27B and surround the respective isolation diaphragms 39 and 40. In turn, the ring 51 is welded as at 53 to an interior surface of the massive ring 50. A retainer ring 55 which surrounds the sensing cell 26 and fits into a provided cavity in the ring is held in place through the use of cap screws 57 that pass through provided openings in a shoulder portion of the ring 50 adjacent to the ring 51. The cap screws 57 are threaded into the ring 55. The ring is welded as at 54 to the annular ring 52. The pressure on the center section is controlled by the cap screws 57. However, the outer edge portions of the ring 50 are of greater axial length than the rest of the clamping assembly and these outer portions bear against surfaces of the end caps 14 and 16 so that when the screws 17 are tightened, the force from the screws 17 is taken by the ring 50 and is not transmitted to the center cell assembly 26. It should also be noted that suitable 0 rings 60 are provided between the end caps 14 and 16 and housing 27. There is also an O ring 61 provided between the outer periphery of the ring 55- and the inner periphery of the cavity on the large ring 50 that the ring 55 slips into. This provides for a seal in both instances.

The cavity volumes in the isolation and sensing chambers are selected so that the sensing diaphragm 28 will bottom before the acting isolation diaphragms 44 or 45 will bottom against the surface defining depression 31. This means that the bottoming action, for example, if pressure is acting on the diaphragm 45, will be against capacitor plate 39 with the measuring diaphragm 28 resting against this capacitor plate before the isolation diaphragm 45 bottoms against the surface of the housing. This insures that the overpressure stop will be positive, and because the deposited capacitor plates 39 and 40 are very stable due to the massive amount of glass fused onto the housing 27, there is no shift in calibration and the unit is not damaged in any way. This same concept of overpressure protection can be used in a pressure sensor which actually senses only one pressure but uses an isolation diaphragm.

All of the bubbles are removed from the oil at the time it is placed in the chambers and before the tubes 34 are sealed. The isolation diaphragms can be stainless steel and have to be compatible with the fluid being sensed through the conduits 18 and 19. The sensing diaphragm itself needs only be compatible with the relatively inert oil or other liquid used for filling the respective chambers 48 and 49. The diaphragm 28 and the housing 27 usually are of the same material to be compatible. The capacitor plates 39 and 40 which are deposited on the glass surface forming the interior sensing chambers 48 and 49 can be nickel or any desired metal. The capacitor plates are relatively thin and form electrical contact with the walls of tubes 34, but are spaced from the metal hOuSing 27.

The glass in the cavities 29 is fused to the surfaces of the housing. It is important that the glass remains under compression while it is cooling to prevent shattering.

There are no friction or sliding members since the glass fills the housing cavity and the capacitor plates are deposited right on the glass. This construction provides a stable assembly which withstands extremely high pressures of both short and long duration without significant effect to the instruments calibration.

One of the problems in capacitive type sensors having an oil filling between isolation diaphragms and the sensing diaphragm, as shown, is the expansion of the oil due to temperature changes. This is the reason for having the measuring or sensing diaphragm bottom out. Thus oil expansion will not affect the pressure at which the diaphragm 28 will bottom out therefore protecting against overpressure damage even if the oil has expanded because of increased temperatures. If the overpressure protection was dependent upon the isolation diaphragms 44 or 45 bottoming out, and the oil expanded, the overpressure stop point could change enough to cause damage to the sensing diaphragm before the isolation diaphragms had bottomed out.

Sensing or readout circuitry usable with the device of the invention can be such as that shown in my application for Transducer Circuitry for Converting a Capacitance Signal to a DC Current Signal, 'Ser. No. 869,608, filed on even date herewith.

What is claimed is:

1. A transducer for use in sensing pressures comprising a housing, first diaphragm means defining a sensing chamber in said housing in cooperation with a housing surface, capacitor plate means on said housing surface and said first diaphragm whereby deflection of said first diaphragm will cause changes in electrical capacity between said capacitor plates, an isolation diaphragm defining an isolation chamber spaced from said sensing chamber, said isolation diaphragm 'having a surface open to a source of fluid to be sensed, fluid passage means between said sensing chamber and said isolation chamber, a substantial- 1y noncompressible fluid filling said isolation chamber, said sensing chamber, and said fluid passage means whereby movement of said isolation diaphragm under pressure within a selected pressure measurement range causes deflection of said first diaphragm, and mechanical support means on said housing positioned to substantially fully support said first diaphragm against deflection when the pressure on said isolation diaphragm exceeds the selected pressure measurement range and to thereby support, through said noncompressible fluid, said isolation diaphragm against further deflection.

-2. The combination as specified in claim 1 wherein metal tube means is provided extending through a wall of said housing and is insulated therefrom, said tube opening to said sensing chamber and being electrically connected to the capacitor plate means on said housing surface, said tube providing means for insertion of said noncompressible fluid into said transducer, said metal tube further serving as electrical connection means to the capacitor plate means on the housing surface.

3. The transducer as specified in claim 1 wherein said housing surface comprises a part spherical surface defined on said housing.

4. The transducer of claim 1 wherein said housing comprises a metal housing and a quantity of glass filling the interior cavity, said glass being fused to the walls of said housing, and said housing surface being an exposed surface defined in said glass.

5. A sensing device comprising a cell housing, an interior cavity defined by cavity surfaces in said cell housing, a sensing diaphragm fixed to said cell housing and sealingly dividing said cavity into first and second chambers, said cell housing further having first and second isolation cavities defined therein, first and second isolation diaphragm means sealing said isolation cavities respectively and defining at least one wall portion thereof, re- Spectively, liquid passage means between said first of said chambers and said first isolation cavity, and separate liquid passage means between said second chamber and said second isolation cavity, a substantially noncompressible liquid filling said isolation cavities, said chambers, and said liquid passage means, and means from separate sources of fluid under pressure opening separately to surfaces of said first and second isolation diaphragms whereby diiferential in pressure on said isolation diaphragms causes relative deflection of said sensing diaphragm from a neutral position, and capacitive sensing means on said interior cavity surfaces and said sensing diaphragm for sensing the deflection of said sensing diaphragm with respect to the walls thereof.

6. The sensing device of claim 5 and cap means on said cell housing, a chamber to open to one of said isolation diaphragms, a support wall, means to fasten said cell housing to said support wall, and means to mount said cap means with respect to said cell housing, said cap means being compressively supported on said massive wall portions before being supported on said cell housing.

7. The sensing device of claim 6 wherein said means to mount said cap means includes a clamp member on an opposite side of said massive wall from said cap means, and means to compressively tighten said cap means and said clamp member together with said massive wall means between said clamp member and said cap means.

8. A pressure transducer comprising a sensing unit having a central cavity defined therein, said cavity being defined by two part spherical surfaces facing each other and joining along a central parting plane, a sensing diaphragm normally lying along the parting plane and fixed to said sensing unit to thereby divide said cavity into two separate sensing chambers, a pair of separated isolation cavities defined in said sensing unit, first and second fluid passage means opening between said first and second isolation cavities and said first and second sensing chambers, first and second isolation diaphragms sealing said first and second isolation cavities respectively, and each having at least a surface portion open to the exterior of said sensing unit, substantially noncompressible liquid means substantially filling the isolation cavities, fluid passage means and chambers, means defining capacitor electrodes on the sensing diaphragm and on the part spherical surfaces defining said central cavity, and means from separate sources of fluid under pressure opening to each of said isolation diaphragms to subject said isolation diaphragms to the respective pressures, whereby differentials in pressures on said isolation diaphragms will act through said noncompressible liquid means and cause corresponding deflection of said sensing diaphragm toward one of the surfaces defining said central cavity, means to enable detection of deflection of said sensing diaphragm comprising capacitor electrodes on the sensing diaphragm and on the surfaces defining said central cavity, and means to sense the changes in capacitance between said electrodes.

9. The combination as specified in claim 8 wherein said sensing unit comprises a metal housing having a central chamber filled with solid insulating material fused to said housing, and said part spherical surfaces being defined at least partially in the solid insulating material, said capacitor electrodes on said part spherical surfaces being only on the portion of said part spherical surfaces defined in said insulating material.

10. The transducer of claim 8 wherein said insulating material comprises glass.

11. The combination as specified in claim 10 wherein separate metal tubular means are provided extending through said glass and are fused thereto, one of said tubular means opening to each of said part spherical surfaces, said tubular means permitting the filling of said noncompressible liquid into the respective sensing chambers, and further being electrically connected to the respective capacitor electrodes on said surfaces.

12. The combination as specified in claim 11 wherein said metal tubular means are positioned in said metal housing prior to the time the glass is fused to said housing and said tubular means.

13. The combination as specified in claim wherein each of said fluid passage means includes a ceramic element fused to said glass and extending into a passageway in the metal walls of said housing, said ceramic element having at least one axial opening defined therein.

14. The combination as specified in claim 8 wherein the liquid volume of said first and second isolation cavities, respectively, is greater than said first and second chambers, respectively.

15. The combination as specified in claim 8 wherein said sensing diaphragm is stretched across said cavity and is welded to the housing around the outer periphery of said housing.

16. A pressure sensing cell assembly of the capacitor type comprising a housing constructed of two metal sections, each of said sections having an internal cavity, said cavities being filled with a hard insulation material such as ceramic material or glass fused to the metal portions defining said cavities, metal tubular member passing through said hard insulation material and out the wall of said housing and being fused to said hard insulation material and insulated from said housing, a central cavity formed in said housing by surface means defined in said hard insulation material, sensing diaphragm means along the parting line between said housing sections and fixed to said housing and sealing said central cavity into first and second sensing chambers, capacitor plate means on said surface means defined in said hard insulation material and electrically connected to said tubular means, respectively, said housing sections having isolation cavities defined therein and spaced from the sensing chambers, isolation diaphragm means sealing said isolation cavities to form first and second isolation chambers, fluid passage means between said first and second isolation chambers and the first and second sensing chambers, respectively, and substantially noncompressible fluid means filling said isolation chambers, said fluid passage means and said sensing chambers, said metal tubular members being sealed after the fluid means is placed in said housing to form electrical lead means.

17. The combination as specified in claim 16 wherein said surfaces of said sensing chamber are positioned to support the sensing diaphragm against increased deflection in either direction of deflection before the isolation diaphragms are supported against deflection.

18. The combination as specified in claim wherein said capacitor plate means on said surfaces defined in said hard insulation material are formed by depositing a thin layer of metallic material on said surfaces defined in said hard insulation material prior to the time said sensing diaphragm is fastened to said housing.

19. A sensing device comprising a cell housing, an interior cavity defined by cavity surfaces in said cell housing, a sensing diaphragm fixed to said cell housing and sealingly dividing said cavity into first and second chambers, said cell housing further having first and second isolation cavities defined therein, first and second isolation diaphragm means sealing said isolation cavities respectively and defining at least one wall portion thereof, respectively, liquid passage means between said first of said chambers and said first isolation cavity, and separate liquid passage means between said second chamber and said second isolation cavity, a substantially noncompressible liquid filling said isolation cavities, said chambers, and said liquid passage means, and means from separate sources of fluid under pressure opening separately to surfaces of said first and second isolation diaphragms whereby differential in pressure on said isolation diaphragms causes relative deflection of said sensing diaphragm from a neutral position, and capacitive sensing means on said interior cavity surfaces and said sensing diaphragm for sensing the deflection of said sensing diaphragm with respect to the walls thereof, said isolation cavities being of larger volume than the respective chambers to which they are connected, and said sensing diaphragm and cavity surfaces being defined so said sensing diaphragm will defiect to position contacting one of the cavity surfaces before the isolation diaphragm causing the deflection is supported.

20. A pressure transducer having a sensing cell of the capacitive type including a capacitive sensing diaphragm supported between two sensing cell sections, a wall member, means mounting said sensing cell to said wall member and compressing said cell sections against said sensing diaphragm at a first stress level, said wall member including support portions, and an inlet housing means, means compressively clamping said inlet housing to said wall member, said Wall support portions mechanically supporting said inlet housing means before the inlet housing means directly contacts said cell to support said inlet housing means without substantially changing the stress on said diaphragm.

21. The sensor of claim 20 wherein said sensing cell comprises an isolation diaphragm forming an outer surface of said cell, and a noncompressible fluid transferring movement between said isolation diaphragm and said sensing diaphragm, and means sealing said inlet housing means adjacent the edge portions of said isolation diaphragm to form a pressure inlet to said isolation diaphragm.

22. The sensor of claim 21 wherein said wall member substantially surrounds said cell means and forms an opening thereto, said inlet housing means closing said opening, and said means compressively clamping said inlet housing to said wall member comprises a second housing section on an opposite side of said sensing cell and supported on an opposite side of said wall member, and means acting on said inlet housing means and said housing section to urge said inlet housing means and said housing section toward each other to thereby clamp said wall member therebetween, said inlet housing means and said housing section both being supported on said Wall member without substantially changing the stress in said diaphragm.

References Cited UNITED STATES PATENTS DONALD O. WOODIEL, Primary Examiner US. Cl. X.R. 73-407 R 

